Data recovery on ISO9660 CDs
ISO9660 is the ‘father’ of CD data disk format. As with all standards it has evolved, typically to what is now as Joliet which from the users point of view has the major benefit of storing files names in upper and lower case, and longer than the rather limited name length of ISO9660
A very valuable feature of 9660 CDs is the ability to be able to add sessions to a disk. Thus if a disk has only 20MB recorded on it, it is possible to add an extra 600MB or so of data, rather than throwing away the disk. However, the original data is no longer accessible to a PC, and so this is the cause of many people losing their important files. Fortunately, CnW Recovery has developed tools to allow all previous sessions to be restored
An ISO9660 disk starts at sector 16 with a volume header, CD0001. (High Sierra uses CDROM and Compact Disc Interactive uses CD-I). If the disk is a Joliet version, then there is a header at sector 17. Each header points to areas of the disk that store the file names in either base 9660 format, or Joliet format - ie long files names in both lower and upper case. The 9660 structure does allow for multiple headers pointing to multiple structures on the disk. A typical third header is for a UDF structure, and thus creating a 9660 / UDF bridge disk that could be read as either 9660 or UDF. The data is stored once, but there are two directory systems on the disk.
There are three possible levels of interchange for ISO9660 disks, and these are based on character sets for file names. For instance, level one is restricted to no more than 8 characters for a file name, and 3 characters for file extension, equivalent to DOS version 1. Level 2 allows longer file names and can be used to interchange between DOS, Macintosh and Unix. Level 3 will allow for interleaved data and audio files.
With 9660 formats, the files are always written sequentially which does assist in recovery, as it is always predictable where files will be. The common problems though are due to scratching and general failures of the disk making it impossible for the standard operating system to read the data. CnW can skip over these problems and so recover data files from the CD, or DVD with only minimal loss of data. A bad scratch will result in loss of data, but could kill the whole disk as far as Windows is concerned. CnW though will recover what is possible. CnW can also recover erased CD-RW and often CDs that have been corrupted and so look like a totally blank CD
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